Wireless networks, such as cellular networks, typically employ radio frequency spectrum allocated by governmental bodies. Governmental bodies allocate the radio frequency spectrum between a number of wireless network operators. Additionally, only certain portions of the radio frequency spectrum is useable for wireless networks. Accordingly, radio frequency spectrum is a scarce and valuable resource that needs to be used as efficiently as possible, which is commonly referred to as spectral efficiency. In cellular networks, frequency reuse, i.e., how often frequencies are reused in different cell sites, is a common measure of spectral efficiency.
One limiting factor on spectral efficiency is signal-to-noise ratio (SNR). Wireless voice communications typically require an SNR above a particular threshold level in order to provide a voice call. Because voice calls only require SNR levels just above the particular threshold level, little benefit is seen by providing an SNR level much higher than the particular threshold level in voice-only wireless networks. Rather, the goal for a voice-only network is to consistently provide an SNR level as close to the particular threshold level as possible in order to minimize the power transmitted to that particular wireless device, as excess power will create unnecessary interference to other wireless devices.
In contrast to voice communications, data communications can typically be provided for a variety of SNR levels, where the greater the SNR, the higher the data rate that can be provided. Because providing high data rates is an area of fierce competition between wireless network operators, these network operators desire to provide the highest possible data rates, while still using their allocated spectrum as efficiently as possible.
Wireless data communications are typically provided by sectorized cells. A sectorized cell is one in which the cell is divided into a number of different geographical sectors, each providing different sets of channels. One frequency reuse scheme for sectorized cells is referred to as universal frequency reuse, which is also referred to as N=1×1 frequency reuse. Universal frequency reuse employs the same frequency, or frequencies, in each sector of every cell site. Universal frequency reuse is currently used in 1×EV-DO, F-OFDM, UMTS and other technologies.
Because universal frequency reuse employs the same frequencies throughout the network, high spectral efficiency is attained. However, this also results in considerable interference in areas where cells overlap, which reduces data rates in these areas. The increased interference affects both overlap boundary areas between different cell sites, as well as sector boundary areas within the same cell site. Universal frequency reuse technologies experience an equal-power boundary in between two sectors, thereby resulting in a low SNR and correspondingly low data rates even when wireless devices are geographically close to the base station. These areas of high interference reduce data rates in a significant portion of the wireless data network coverage area, thereby impacting the end user data experience.
Another frequency reuse scheme assigns three frequencies to each cell site, with each of the three sectors employing a different frequency, which is commonly referred to as a N=1×3 frequency reuse. Adjacent cells are arranged such that the sectors of adjacent cells which overlap each use different frequencies. Although this technique improves the SNR in areas where cell boundaries overlap, spectral efficiency is reduced compared to the universal frequency reuse because frequencies are used only ⅓ as often.
Yet another frequency reuse scheme is to allocate three 1.25 MHz carriers to each sector, with each of the carriers being allocated at a different power level within the sector. For example, the highest power carrier in sector 1 would be assigned to sector 2 with a power level approximately 6 db lower than in sector 1, and to sector 3 with a power level approximately 12 db lower. The other carriers are similarly staggered over the sectors. Adjacent cells are designed such that areas of overlap have different frequencies being transmitted with the highest power level, which results in reduced interference in the overlap regions between cells. However, near the center of the cell site, this results in the third, lowest power carrier experiencing average SNR levels several dBs worse than the SNR provided by the two higher-power carriers. Additionally, wireless devices near the center of the cell site are forced to be served on the lowest-power carrier, which degrades the overall sector throughput.